Uncharged Surface Research Articles

Surface Complexation on TiO 2: II. Ternary Surface Complexes: Coadsorption of Cu(II) and Organic Ligands (2,2′-Bipyridyl, 8-Aminoquinoline, and o-Phenylenediamine) onto TiO 2 (Anatase)

The simultaneous adsorption of Cu 2+ and organic ligands L ( L: 2,2′-bipyridyl (BIPY), 8-aminoquinoline (AQ), and o -phenylenediamine (OPDA)) in 0.1 M (K)NO 3 or 0.1 M (Na)ClO 4 solutions at TiO 2 was investigated at 298.2 K. The experiments were performed by alkalimetric titrations. The experimental results for TiO 2 are explained by assuming the formation of ternary surface complexes according to the general equation 〉SH 2 + pH + + q Cu 2+ + rL = 〉SH 2+pCu qL (p+2q)+ r , where 〉 SH 2 denotes an uncharged surface site. The stability of these complexes as defined by the stability constants β S p,q,r(int) = [〉SH 2+pCu qL (p+2q)+ r] [〉SH 2][H +] p[Cu 2+] q[L] r e (p+2q)F↓/RT was found to be: (a) for L = BIPY, log β S 0,1,1,(int) = 11.37, log βS -1,1,1,(int) = 6.00, log β S -2,1,1,(int) = -1.57, (b) for L = AQ, log β S 0,1,1,(int) = 9.80, log β S -1,1,1,(int) = 4.11, log β S -2,1,1,(int) = -2.78, and (c) for L = OPDA, log β S 0,1,1,(int) = 8.52, log β S -1,1,1,(int) = 1.86, log β S -2,1,1,(int) = -4.75. The possible structures of the postulated complexes are discussed.

Structure of the metal-electrolyte solution interface: Theoretical results for simple models

Theoretical results are given for electrolyte solutions in contact with uncharged metallic surfaces. The metal is represented by a jellium slab and the electron density distribution across the interface is obtained using density functional theory. The solution ‘‘structure’’ is found by solving the reference hypernetted-chain approximation. The two interfacial phases interact electrostatically and fully self-consistent electron density distributions and wall-particle molecular correlation functions are calculated. The electron densities, wall-induced solution structure and the electrostatic potential drop across the interface are discussed in detail and compared with the inert wall case. It is found that a highly ordered region exists near the metal surface. The dipoles are strongly ordered by the metal field and this solvent structure effectively dictates the ion distributions. The direct ion-metal interactions are found to be less important.

Quantum chemical study of the adsorption of an H2O molecule on an uncharged mercury surface

Despite the fact that attempts were made to describe the interaction of a single H2O molecule with a mercury surface, using both semi-empirical and ab initio quantum chemical calculations, reliable microscopic information on the Hg | H2O interface is still lacking. In this work, non-empirical quantum chemical calculations were carried out to study water molecule adsorption on an uncharged mercury electrode. The mercury surface was modelled by a cluster Hgn, with n = 6, 7. The effect of electronic correlation plays an important role. An “on-top” position of the H2O molecule with the dipole moment pointing away from the surface reveals an adsorption energy minimum (ΔEads) of −38.5 kJ mol−1. The dipole reorientation energy was estimated to be 21.8 kJ mol−1. According to our results, the dependence of ΔEads on the tilt angle has no limit. Analysis of the chemical binding between the cluster and the H2O molecule shows the electrostatic nature of the binding. The mean field approximation was applied to describe the interaction between the adsorbed H2O molecules in a monolayer. The results were in agreement with experimental data.

Creepage discharge propagation in air and SF/sub 6/ gas influenced by surface charge on solid dielectrics

Influence of surface charging on creepage discharge propagation in air and SF/sub 6/ gas was investigated. The authors have employed a surface charging system using corona discharges, which permitted the deposition of surface charge on solid dielectrics with a given density distribution. By applying a lightning impulse across the charged surface of polyester film, they measured the discharge extension length as a function of surface potential generated by the surface charge. It was found that the impulse creepage discharges along the charged surface under a potential of 2 to 3 kV, extended a distance which was maximum 6/spl times/ longer than the case of the uncharged surface both in air and SF/sub 6/ gas. Experimental results were also discussed from the viewpoint of the specific capacitance, permitting their application to actual HV apparatus such as GIS.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Investigation of capacitive coupling voltage contrast using a specimen charging model

Abstract The capacitive coupling voltage contrast effect has been simulated using a versatile charge simulation program. Simulation shows that the potential distribution of the irradiated passivation surface depends on the size and the shape of the beam profile. When the width of the beam spot is equal to or less than the width of the buried electrode and the electrode voltage is positive, a negative charge ring is formed around the irradiated spot. As a result of this charge ring, the irradiated spot above an electrode with non-positive voltage need not charge to a positive equilibrium potential. Differential charging of passivation surface enhances the lateral field on the surface. The field is highest during voltage switching, its amplitude depends on the voltage swing on the electrode and can be several times stronger than the field existing on the uncharged surface. The field strength reduces slightly at equilibrium.

Enthalpy Changes for Brønsted Acid-Base Reactions on Silica

Reaction enthalpies for Brønsted acid-base reactions on pyrogenic silica were determined by titrating an aqueous suspension of solid particles with standard acids and bases in an isoperibol calorimeter. Formation of negative surface charge by proton desorption from a hydroxyl functional group is endothermic. In the limit of an uncharged surface, the reaction enthalpies range from 0-10 kJ/mol of charge. This range for silica is smaller than that measured on other oxide minerals (15-30 kJ/mol) by other researchers. The reaction enthalpies on silica become increasingly endothermic by approximately 1-2 kJ/mol of charge for each μC/cm2 change of charge densities as the pH (and total charge density) increases from the point of zero charge (p.z.c.). These enthalpy changes are interpreted with a modified form of the Bragg-Williams model of interactions among adsorbed species, yielding mixing parameters and activity coefficients for the surface complexes.

Adsorption of ionic block copolymers: self-consistent-field analysis and scaling predictions

The adsorption of ionic diblock copolymers on an uncharged surface is investigated. One block, the «anchor», consists of N A uncharged, adsorbing A segments, whereas the «buoy» block has N B segments which carry a fixed charge and are nonadsorbing. Upon adsorption these molecules form a layer that resembles a brush of B segments. In contrast with a usual brush, however, the molecules in the adsorbed layer are in equilibrium with those in solution. Consequently, the chain density of the brush is not fixed; its value is determined by the dynamic equilibrium between polymer adsorption and desorption

Field-dependent fibroblast orientation on charged surfaces is independent of polarity and adsorbed serum proteins

Electrets were used to induce a dipolar charge on fluorinated ethylene propylene (FEP) and bacteriological grade polystyrene (PS) films. The serum protein surface concentration adsorbed on FEP and PS from cell-free media was dependent on both the magnitude and the polarity of the surface charge density. Chick embryo fibroblasts were cultured on charged and uncharged FEP surfaces, and cellular orientation and biosynthetic activity (protein synthesis) were determined. The orientation of fibroblasts was found to be significantly dependent on the magnitude of the surface charge but independent of its polarity. The biosynthetic activity of fibroblasts was observed to be dependent on both magnitude and polarity of the surface charge density.

The effects of 1 kW class arcjet thruster plumes on spacecraft charging and spacecraft thermal control materials

Arcjet thrusters are soon to be used for north/south stationkeeping on commercial communications satellites. A series of tests was performed to evaluate the possible effects of these thrusters on spacecraft charging and the degradation of thermal control material. During the tests the interaction between arcjet plumes and both charged and uncharged surfaces did not cause any significant material degradation. In addition, firing an arcjet thruster benignly reduced the potential of charged surfaces to near zero. >

Submicron-range attraction between hydrophobic surfaces of monolayer-modified mica in water

Very-long-range attraction extending to a separation close to 300 nm was observed between uncharged mica surfaces that arc modified by hydrophobic layers of a polymerized ammonium amphiphile. The force distance profile in pure water is expressed by an exponential function composed of intensity parameter and decay length. Their values determined from the deflection method are 1.7±0.5 mN/m and 62±4 nm, respectively. The same parameters obtained from the jump-in method, 0.59 mN/m and 72 nm, agree within the experimental error

Escape depth of secondary electrons from electron-irradiated polymers

Measurements on polymers (Teflon, FEP and Mylar) have shown that the secondary electron emission from uncharged surfaces exceeds that from surfaces containing a positive surface charge. The reduced emission of charged surfaces is due to recombination between electrons undergoing emission and trapped holes within the charged layer. During the experiments the surface of the material was kept at a negative potential to assure that all secondary electrons reaching the surface from within the material are actually emitted. An analysis of the results yielded the maximum escape depth of the secondary electrons, and showed that the ratio of the maximum escape depth of the secondaries from Mylar to the maximum escape depth from Teflon is almost the same as the ratio of the corresponding second crossover energies of these polymers. >

On the Electrostatic Component of the Adhesional Interaction between Solid Substrates and Langmuir Monolayers

AbstractThe work of adhesion between Langmuir monolayers and solid surfaces in a hydrophilic Langmuir‐Blodgett configuration has been measured as function of the surface charge densities σ of the contacting surfaces. It was derived from the equilibrium meniscus height which was measured with the transfer fluorescence microscope. The surface charge density of the contacting surfaces was varied by changing the subphase pH. With both surfaces negatively charged (dimyristoylphosphatidylethanolamine onto SiO2‐substrates) and low charge densities (σ &lt; 1 e−/100 Å2)) we observe a decrease of the adhesion with increasing charge density. At high charge densities (dimyristoylphosphatidic acid onto SiO2‐substrates, σ &lt; 1 e−/100 Å2) the work of adhesion is hardly affected by charge density variations, as theoretically predicted for electrostatic double layer forces. Systems with only one surface of varying negative charge density (dipalmitoylphosphatidylcholine onto SiO2) unfold the predicted decrease of the work of adhesion with increasing charge density. Control experiments with varying pH and uncharged surfaces (dipalmitoylphosphatidylcholine onto polymethylmethacrylate surfaces) show a pH‐independent work of adhesion and thus corroborate that the measured behaviour is caused by the electrostatic double layer interaction.

In-Situ Electro-Charging for Friction Reduction and Wear Resistant Film Formation

A technique has been developed to form friction-reducing and antiwear films in-situ by means of an electric field applied between rubbing metal surfaces. Using this in-situ charging technique, the effectiveness of zinc organodithiophosphate (ZDP) additives as antiwear and friction reducing agents can be enhanced by electrochemical reactions which form surface coatings. The coatings formed on sliding surfaces compared with uncharged surfaces reduce friction by up to 35 percent. In addition, during the sliding process, the films formed on the surfaces are replenished and protect the rubbing surfaces from wear. A steel plate subjected to rubbing and in-situ charging for four hours had a smooth surface and a yellowish reaction film; the one without charging was rougher and no yellowish reaction film was formed. The improved tribological characteristics are attributed to an increase in the electrochemical reactivity of the ZDP-mineral oil blends and the formation of phosphate or sulfate films on the metal surf...

Multisite proton adsorption modeling at the solid/solution interface of (hydr)oxides: A new approach: II. Application to various important (hydr)oxides

At the solid/solution interface of metal (hydr)oxides various types of O(H) and OH(H) groups are present, which differ in the number of coordinating metal ions. The σ 0-pH curves of metal (hydr)oxides are strongly determined by the composition and the relative extent of the various crystal planes of (hydr)oxides. The charging behavior is discussed for gibbsite (Al(OH) 3), goethite (FeOOH), hematite (Fe 2O 3), rutile (TiO 2), and silica (SiO 2). New experimental σ 0-pH data for goethite and gibbsite are presented. Several important (hydr)oxides exhibit crystal faces which do not develop surface charge over a relatively wide pH range. An uncharged crystal face may be due to the presence of surface groups which are not reactive (inert) in the pH range under consideration, like the 001 face of gibbsite and the 0001 face of hematite, or caused by the presence of two types of interacting charged surface groups of which the charge of one type is fully compensated by the other like at the 100 face of goethite. The charging behavior of silica and the 001 face of gibbsite is determined by one type of reactive surface group with a large Δp K for the consecutive protonation steps. The crystal structure imposes the presence of uncharged surface groups and this results in a quite different shape of σ 0-pH curves for gibbsite and silica in comparison with the commonly observed σ 0-pH curves of metal (hydr)oxides. The MUltiSIte Complexation (MUSIC) model as developed by T. Hiemstra, W. H. Van Riemsdijk, and G. H. Bolt, ( J. Colloid Interface Sci. 132 (1989)) leads to a rather good prediction of σ 0-pH curves for various metal (hydr)oxides using predicted affinity constants for the various types of surface groups and Stern layer capacitance values and pair formation constants estimated from the literature.

Molecular dynamics simulation of water beween two ideal classical metal walls

We have simulated a slab of water with two-dimensional periodic boundary conditions between two metallic walls. The entire compliment of charges, arising from periodic reproductions and from classical images in the metal, are included explicitly by mapping onto a problem with three-dimensional periodicity which is handled by usual Ewald summation methods. Results are presented for charged and uncharged surfaces, permitting an estimate of the differential capacitance arising from the layer of water near the walls. The estimate is about a factor of 2 smaller than the observed differential capacitance of metal–aqueous electrolyte interfaces.

Anharmonic and screening corrections to the melting of a two-dimensional charged colloidal crystal.

The Kosterlitz-Thouless-Halperin-Nelson-Young theory of melting is applied to a two-dimensional charged colloidal crystal. The renormalization of elastic constants due to anharmonic and dislocation pair-screening effects is calculated. For colloidal spheres (``polyballs'') suspended in a thin film of water confined between two glass plates whose surfaces are highly charged, as in recent experimental configurations, the corrections to the harmonic results simply require an altered effective polyball charge to reproduce virtually the identical melting curve of the harmonic approximation (which agrees well with experiment). The effect on the melting curve of added salt is also calculated; the corresponding experiments have yet to be performed. For a film between uncharged surfaces the anharmonic correction again softens the lattice significantly. Dislocation pair screening causes an additional but minor reduction of the elastic constants. Neither correction eliminates a nonmelting behavior characteristic of the two-dimensional screened Coulomb system in this configuration.

Adsorption of human beta 2-microglobulin at a water/mercury interface.

Adsorption of human beta 2-microglobulin from a neutral solution of 0.15 M NaCl on a mercury surface was studied at 25 degrees C by measurement of the differential capacity of the electrical double layer. From the diffusion-controlled adsorption kinetics, the surface concentration and hence the area occupied by the adsorbed beta 2-microglobulin molecule were determined at various potentials of the mercury surface. The results indicate unfolding or flattening of beta 2-microglobulin molecules adsorbed in particular on the electrically uncharged surface. The extent of this interfacial conformational rearrangement was reduced with growing positive or negative surface charge density.

Interaction of planar double layers in the modified Gouy-Chapman approximation

The force between two charged planar surfaces containing an electrolyte solution is calculated. The calculation is done for a 1-1 electrolyte with size-asymmetric ions using a Modified Gouy-Chapman theory. It is shown that at least part of the explanation for the sharp rise in the force between charged surfaces at small separations seen in experimental data may be related to finite-sized ion effects in the double layer. An interesting effect of size-asymmetric ions is the prediction of a force between uncharged surfaces.

Specific adsorption of ascorbate ions on a mercury electrode in the presence of potassium perchlorate

The specific adsorption of ascorbate ion (AH −) on a mercury electrode in aqueous solutions of 10 −2 M KClO 4 has been determined from electrocapillary and capacity measurements. The ascorbate anion removes the perchlorate ion from the interface when its concentration is greater than 1 × 10 −3 M. The adsorption of ascorbate anions can be fitted by a Frumkin isotherm with a value of −2.4 for the α-parameter. The standard Gibbs energy of adsorption is a linear function of the surface charge and the Δ G° value at zero charge is −20.3 kJ mol −1. The inner-layer capacity is analyzed into its components. Comparison of the results obtained for AH − with those reported for its conjugated acid, vitamin C, shows that the interaction energy between the electrode and both species is very close at the uncharged mercury surface. At zero charge, the anion is adsorbed less than the acid. The difference in behaviour at the interface between the anion and the acid is discussed.

ADHESIVE INTERACTIONS BETWEEN THE TUBE FEET OF A STARFISH,LEPTASTERIAS HEXACTIS, AND SUBSTRATA

The tube feet of Leptasterias hexactis adhere to and release from substrata by chemical interactions. In our laboratory these podia adhered to substrata coated with the ubiquetous anionic saccharide films produced by marine bacteria. Podia also attached to moderately anionic glass, but not to uncharged surfaces. The adhesive epithelia of tube feet labeled heavily with ruthenium red, indicating they were anionic. Tube feet secreted footprint films that bound crystal violet, a cationic dye. Trypsin removed the films. Adhesion to marine surfaces was prevented by 300 units/ml of heparin, a glycosaminoglycan (GAG) that may have competitively inhibited the glue from binding exosaccharide marine films. Lectins that bind bacterial exosaccharides did not inhibit attachment. We propose that tube-foot attachments are nonspecific ionic interactions established by secreted proteinaceous films and released when secreted GAG's compete with the tube-foot epithelium for sites on the film. This system agrees with the duo-gland model for adhesion and deadhesion.